1. Field of the Invention
The technical field of the invention relates to the production of tissue paper on a corresponding paper-making machine in which more particularly a through air drying (TAD) zone is provided. In this TAD zone a special imprinting fabric is employed.
2. Prior Art
The sheet formation of the paper and the three-dimensional structuring of an already formed moist fiber felt which is still deformable, however, due to a remaining high water content, is usually done on backing woven fabrics stemming from textile weave processes.
Three-dimensionally structuring a moist paper web by forming zones of low density framed by dense zones is undertaken in modem tissue making machines in the course of predrying the sheet in a predrying section upstream of the yankee cylinder. Predrying the paper web occurs on the backing fabric by convection in forcing hot air through the paper web located on the backing fabric. This is termed through air drying (TAD).
Three-dimensional structuring is usually implemented in three steps mostly sited separately in sequence. The first step involves deflecting the fibers in the Z direction into the structuring depressions in the backing fabric made available by the TAD imprinting fabric systematically distributed over the surface area of the backing fabric contacting the paper. Deflecting the fibers in the Z direction is prompted by a flow of air and water, vacuum-assisted by one or more vacuum boxes arranged on the side of the backing fabric opposite the side in contact with the paper.
Deflecting the fibers in the Z direction into the interior of the depressions results in zones of reduced density in the paper sheet which are termed pillows. These zones of reduced density arranged in a pattern are dried in a second step on or in the interior of the backing fabric by the air flowing therethrough of one or more TAD cylinders and thus set in the existing distribution of the fibers, i.e. xe2x80x9cfreezingxe2x80x9d the fiber distribution status.
Then, in a third step partial compression of the predried fiber felt takes place by pressing the backing fabric with the predried web of paper located thereon with the aid of a compression roller against the surface of the yankee cylinder. Compression of the paper web occurs in the raised portions of the backing fabric which may be formed by both warp and weft wires in the predefined portions of the backing fabric surface. The fibers located in the depressions of the backing fabric receive no compression. TAD imprinting fabrics as the backing fabric represent a special type of fabric comprising typical structurizing properties by their weave, choice of wire as regards material, diameter, cross-sectional shape and after-treatment, for example, heat setting and grinding of the surface.
Paper-making-machine fabrics are known for example from WO 96/04418, DE-OS 30 08 344, EP 0 724 038 A1.
The technical problem (object) of the invention involves providing a paper-making-machine fabric which is suitable and configured, as regards a tissue paper having an enhanced three-dimensional surface structure in the form of a sequence of pillows and pockets, to achieve a tissue paper of enhanced visual appeal, improved softness and greater volume in conjunction with an improved water absorption and better feel.
This problem is solved more particularly by the features of claim 1.
Due to the solution in accordance with the invention a paper-making-machine fabric is provided in which exceptionally deep pockets are provided with the result that more particularly in the TAD zone with this paper-making-machine fabric a paper and, more particularly, a tissue paper is producible which features an exceptionally large three-dimensional structure as regards an increase in the specific volume which makes the paper appear particularly fluffy and features in addition to exceptional softness also exceptionally good water absorption. In addition to this, an enhanced similarity to a woven structure and thus to the look and feel of cloth is achieved.
With the paper-making-machine fabric as described, a paper structure is producible having a large number of pillow-like zones of reduced density provided systematically distributed over the full surface area of the fiber felt. The extent of the pillow-like zones of reduced density in the Z direction, i.e. their thickness, is a maximum relative to their size in surface. Each low-density pillow-like zone is evidently separated from its adjacent pillow-like zone by a line-type frame of increased density, this line-type frame being formed continuously or discontinuously by interruptions. The line portions visually appearing continuous are characterized by a greatly increased, even density as compared to the low-density of the pillow-like zones. If the line portions are interrupted, the line portions in the region of this interruption feature a low density as compared to that of the continuously appearing line portions which, however is significantly higher as compared to that of the pillow-like zones.
The line-type frames dictate the surface-area extent of the pillow-like zones. The entirety of the pillow-like zones with their line-type frames furnishes a visually obvious macroscopic distribution pattern which is typical for TAD imprinting fabric used for structuring and its weave and finish.
In this arrangement the three-dimensional structure produced in the fiber felt with its typical pattern is the mirror image of the three-dimensional structure and distribution pattern of the fabric used in production. More particularly when employing TAD and more particularly when increasing the density as mentioned above is undertaken at the drying cylinder the tissue papers produced in accordance with the invention feature, as compared to non-structured tissue papers produced conventionally, a significantly increased specific volume with added fluffiness as well as an enhanced absorption capacity for liquids, especially water.
Also as compared to conventional TAD paper-making-machine fabrics the TAD paper-making-machine fabrics in accordance with the invention produce a paper having a significantly increased specific volume, added fluffiness and improved liquid absorption capacity.
Further aspects read from the sub-claims. A further increase in the depth of the pockets is achievable by the features of claim 2. From the remaining sub-claims a series of example embodiments materializes.